Method for Reducing Negative Effects of Natural Pitch Contaminants in Both Pulping and Papermaking Operations

ABSTRACT

The instant invention relates to a method for inhibiting pitch deposition on pulping and papermaking equipment or machinery comprising adding to a pulp slurry containing pitch an effective amount of polymer dispersion comprising a component A and a component B, wherein component A being a homopolymer and/or copolymer of acrylic acid and/or its alkyl esters, or methacrylic acid and/or its alkyl esters, styrene and/or methylstyrene, vinyl acetate, itaconic acid, glycidyl methacrylate, 2-hydroxyalkyl (meth)acrylate, methacrylamide, N-hydroxyethyl(meth)acrylamide, dimethacrylate monomers, 1,3-butylene glycol dimethacrylate, ethylene glycol dimethacrylate, diethylene glycol dimethacrylate, propylene glycol dimethacrylate, dipropylene glycol dimethacrylate, 4-methyl-1,4-pentanediol dimethacrylate, divinylbenzene and/or trivinylbenzene, and component B being an aqueous solution of a styrene copolymer with acrylic acid, maleimide and/or maleic anhydride.

The invention relates to resin and pitch control agents, which are aqueous, non-film forming, polymer dispersions and to a process for preventing the deposition of pitch particles in cellulosic pulp suspensions, by use of such polymer dispersions.

Cellulosic pulps contain a considerable proportion of organosoluble matter which is generally referred to as resin or pitch. The resins are extracted from the wood during the pulping process and constitute a significant nuisance in cellulosic suspensions because the resin particles are sticky, tend to agglomerate and form adherent deposits on the pulping and papermaking machinery. The removal of water during papermaking is normally carried out using a type of fabric mesh, commonly referred to as machine wires or felts. Resin or pitch deposits clog and block the small openings in the fabrics inhibiting drainage and causing sheet defects, such as holes in the finished paper. Deposits which accumulate on the internal surfaces of pulp and backwater chests can suddenly be released and displayed as resin lumps in the paper sheet. Larger lumps can break the paper sheet in the machine, leading to loss of production.

For years there have already been products supplied as passivating agents for treating pulp contaminants such as resin or pitch. These dissolved products are intended to make the surface of the tacky impurities more hydrophilic and hence keep them more wettable, thereby reducing the affinity for hydrophobic surfaces. Hydrophobic surfaces are present on, for example, wires, felts and rollers; hydrophobizing is boosted further by coating, with sizing agent or defoamer, for example, thereby further promoting the attachment of pitch.

In certain cases, resins and pitch do not cause any problems in papermaking, if they do not agglomerate. To prevent agglomeration, various methods are known for chemically modifying the pitch particles that have remained in the stock stream and the adsorption thereof on support materials, such as machine wires.

In the context of these problems, the procedures below have been adopted in practice, but lead only to partial success.

On the one hand, dispersion may take place, with the aim of changing the charge on the pitch by means of anionic and nonionic dispersants. This forms colloidal, anionically charged or nonionic particles which counteract agglomeration and deposition. The wetting properties of the dispersant are very important in this case, since the pitch is hydrophobic.

Alternatively, according to the literature, the tack of the pitch can be reduced in the following ways:

-   -   Fixing of the strongly anionic contaminants by means of strongly         cationic fixatives (formation of what are called polyelectrolyte         complexes; the reaction product then adsorbs on the anionic         fiber).     -   Absorption on pigments of high specific surface area (e.g.,         talc, modified clay, mica, smectite, bentonite), often with         subsequent flocculation by means of polymers in order to bind         separable macroflocs.     -   Enveloping (masking) with nonionic hydrophilic polymers         (polyvinyl alcohol) or zirconium compounds, more particularly         zirconium acetate and ammonium zirconium carbonate.

Known strongly cationic fixatives include polyethyleneimine (PEI), polydiallyldimethylammonium chloride (polyDADMAC), polyvinylamine (PVAm), polyaluminum chloride (PAC), polyacrylamide (PAAM), polyamine, etc. The sphere of action of fixatives extends from about 1 nm to 50 micrometers in terms of the particle size of the pitch, depending on the nature and modification of the chemicals used.

Materials with a low surface energy (wires, felts, roller surfaces) exhibit a more hydrophobic behaviour and therefore possess a high affinity for hydrophobic compounds, such as resins and pitch, thereby resulting in contamination of the wires and hence to defects and/or reduction in the dewatering performance of felts.

Adsorbents used are, in particular, various types of talc with specific surface modifications and particle-size distribution, which on account of their hydrophobic and organophilic surface are capable of attaching to adhesive constituents and entraining them with the paper. Particles of adhesive encapsulated in this way have less of a tendency to deposit on hot machinery parts.

Protein solutions are also employed as agents for masking sticky impurities.

The pitch agglomerates tend to deposit on machinery parts, wires, cloths, drying cylinders, and this consequently leads to marks, holes, and instances of web sticking, and consequently to breakages in the wet section and drying section in the course of winding and rewinding or in the course of printing.

DE-102009035884.6/EP 2 462 278 by Clariant discloses a method for reducing negative effects of adhesive synthetic contaminants in systems of substances comprising waste paper. In waste paper the main problem are the pitch agglomerates (stickies) which lead to a deposit on the machinery parts.

In contrary in the process for producing cellulosic pulp suspensions the negative effects are caused by natural pitch contaminants in both pulping and papermaking operations. These contaminants tend to deposit during the production on the cellulosic material and lead to ugly black spots.

In order to prevent resin deposits talc has been known in the prior art to prevent and control pitch deposits. Using talc to control pitch deposits, however, has certain disadvantages. For instance, the system is highly sensitive to shear. Talc, moreover, has poor retention properties and frequently causes clogging of the felts. Talc may adversely affect resin sizing, and stabilizes foam. The two inorganic products, talc and bentonite, require laborious dispersion.

There continues to be a need for improvement in reducing the tackiness of natural pitch and resin particles.

Surprisingly, the tackiness of pitch can be reduced considerably through the use of specific polymer dispersions.

The invention provides an aqueous polymer dispersion and the use thereof in a method for reducing sticky contaminants in the processing of wood pulp and in the papermaking procedure, which involves adding an aqueous polymer dispersion comprising a component A and a component B for passivating and detackifying the pitch particles, component A being a homopolymer and/or copolymer of acrylic acid and/or its alkyl esters, more particularly its methyl, ethyl, butyl, isobutyl, propyl, octyl, decyl, 2-ethylhexyl esters;

-   or methacrylic acid and/or its alkyl esters, more particularly its     methyl, ethyl, butyl, isobutyl, propyl, octyl, decyl, 2-ethylhexyl     esters; -   styrene and/or methylstyrene; -   vinyl acetate; -   itaconic acid; -   glycidyl methacrylate; -   2-hydroxyalkyl (meth)acrylate; -   methacrylamide; -   N-hydroxyethyl (meth)acrylamide dimethacrylate monomers, such as,     for example, 1,4-butylene glycol dimethacrylate, 1,3-butylene glycol     dimethacrylate, ethylene glycol dimethacrylate, diethylene glycol     dimethacrylate, propylene glycol dimethacrylate, dipropylene glycol     dimethacrylate, 4-methyl-1,4-pentanediol dimethacrylate; -   divinylbenzene and/or trivinylbenzene     and component B being an aqueous solution of a styrene copolymer     with acrylic acid, maleimide and/or maleic anhydride.

Component A is a hydrophobic homopolymer and/or copolymer of the above-stated monomers having a very high glass transition temperature or softening temperature (Tg), preferably methyl methacrylate or styrene. The glass transition temperature of A is preferably above 70° C., more particularly above 90° C., very preferably above 100° C.

Component B is a styrene copolymer with (meth)acrylic acid, maleimide and/or maleic anhydride. Component B is preferably a copolymer of styrene and acrylic acid. Component B preferably has a molecular weight of between 3000 g/mol and 15 000 g/mol, more particularly 3000 and 7000 g/mol.

Particularly preferred is an aqueous dispersion with particle sizes of less than 150 nm, preferably less than 120 nm.

The aqueous polymer dispersion may be applied in combination with calcium and or magnesium salts, often naturally occurring in the processing water. Hardness salts insolubilise component B, leading to the de-stabilisation of the tiny emulsion particles. The agglomerated emulsion particles are now more hydrophobic and associate readily and preferentially with any pitch particles in the pulp. The harder emulsion particles reduce the tackiness of the pitch and increase the softening temperature. Hard agglomerates show much less tendency to deposit on machinery.

Where water hardness levels are very low, there may not be sufficient electrolyte to initiate de-stabilisation of the emulsion particles. The aqueous polymer dispersion may therefore be optionally applied in combination with component C, a cationic fixative, which promotes coagulation of the emulsion particles in the cellulosic fibre slurry. Component C is preferably selected from the following group:

-   polyethyleneimine (PEI), polydiallyldimethylammonium chloride     (polyDADMAC), polyvinylamine (PVAm), polyaluminum chloride (PAC),     zirconium salts, polyacrylamide (PAAM), polyamine and     polyamideamine.

The present invention allows Component C to be added during pulp or paper manufacture, either before, after or together with the aqueous polymer dispersions. When Component C and aqueous polymer dispersions are pre-mixed before being added to the fibrous slurry, the particle size increases and the process of destabilisation is initiated. This premature destabilisation is described with the term “pre-crashing”.

It is not essential but preferential to dilute both components before combining them in a pre-crashing application. For aqueous polymer dispersions of the present invention, a dilution of 1 to 20% (based on dry content) is preferred, more preferably 1 to 5%. For Component C, a dilution of 1 to 10% (based on dry content) is preferred, more preferably 1 to 5%. The ratio of the diluted components is controlled using individual dosing pumps and, immediately after blending, the combined components are passed through a static mixer and then into the suction side of a pulp transfer pump, in order to facilitate efficient distribution within the fibrous slurry.

In order to boost the efficiency of the polymer dispersion of the invention and its stability, it is further possible to add a further component D optionally in the form of a surfactant.

Further to components A, B, and/or D, the polymer dispersion comprises water (component E).

In one preferred embodiment the aqueous dispersion comprises

-   2% to 50%, preferably 5% to 30% of component A, -   1% to 30%, preferably 3% to 10% of component B, -   0% to 0.3%, preferably 0% to 0.2% of component D, and -   96% to 17.7%, preferably 90% to 45% of water (component E).     All percentages here relate to % by weight.

In the presence of Ca²⁺, the aqueous dispersion constitutes a self-coagulating nanodispersion. The polymer dispersion of the invention attaches to the hydrophobic sticky particles, incorporating them into the precipitating polymer dispersion and thus detackifying them (FIG. 1).

EXAMPLES Example 1 (Version with Methyl Methacrylate)

A 2 l reactor with stirrer and reflux condenser was charged with 739.5 g of deionized water and 419.3 g of 25% strength solution of styrene-acrylic acid copolymer, this initial charge then being heated to 85° C. with stirring under a nitrogen atmosphere.

Feed stream I:

-   384.8 g of methyl methacrylate     Feed stream II: -   1.9 g of ammonium peroxodisulfate -   136.3 g of deionized water

When an internal temperature of 85° C. had been reached, feed stream I and feed stream II were metered continuously into the polymerization batch via two separate feeds, beginning simultaneously, over a period of 3 h 30, with stirring and retention of the reaction temperature. The pumps were flushed with 318.2 g of deionized water. After the end of both feed streams, the system was left to after react at the reaction temperature for a further 25 minutes. After that, the reaction mixture was cooled to room temperature and filtered on a filter having a mesh size of 160 μm.

The characterization of the copolymer obtained, in terms of solids content (SC) and average particle size (D), is given below:

-   SC=24.1% -   D=53 nm

Example 2 (Version with Methyl Methacrylate+Crosslinker)

A 2 l reactor with stirrer and reflux condenser was charged with 739.5 g of deionized water and 419.3 g of 25% strength solution of styrene-acrylic acid copolymer, this initial charge then being heated to 85° C. with stirring under a nitrogen atmosphere.

Feed stream I:

-   370.9 g of methyl methacrylate -   19.5 g of glycidyl methacrylate     Feed stream II: -   1.9 g of ammonium peroxodisulfate -   136.3 g of deionized water

When an internal temperature of 85° C. had been reached, feed stream I and feed stream II were metered continuously into the polymerization batch via two separate feeds, beginning simultaneously, over a period of 3 h 30, with stirring and retention of the reaction temperature. The pumps were flushed with 318.2 g of deionized water. After the end of both feed streams, the system was left to after react at the reaction temperature for a further 25 minutes. After that, the reaction mixture was cooled to room temperature and filtered on a filter having a mesh size of 160 μm.

The characterization of the copolymer obtained, in terms of solids content (SC) and average particle size (D), is given below:

-   SC=24.9% -   D=40 nm

Example 3 (Version with Methyl Methacrylate+Second Crosslinker)

A 2 l reactor with stirrer and reflux condenser was charged with 740 g of deionized water and 419 g of 25% strength solution of styrene-acrylic acid copolymer, this initial charge then being heated to 85° C. with stirring under a nitrogen atmosphere.

Feed stream I:

-   370 g of methyl methacrylate -   19 g of ethylene glycol dimethacrylate     Feed stream II: -   2 g of ammonium peroxodisulfate -   136 g of deionized water

When an internal temperature of 85° C. had been reached, feed stream I and feed stream II were metered continuously into the polymerization batch via two separate feeds, beginning simultaneously, over a period of 3 h 30, with stirring and retention of the reaction temperature. The pumps were flushed with 318 g of deionized water. After the end of both feed streams, the system was left to after react at the reaction temperature for a further 25 minutes. After that, the reaction mixture was cooled to room temperature and filtered on a filter having a mesh size of 160 μm.

The characterization of the copolymer obtained, in terms of solids content (SC) and average particle size (D), is given below:

-   SC=25% -   D=40 nm

Example 4 (Version with Styrene)

A 2 l reactor with stirrer and reflux condenser was charged with 739.5 g of deionized water and 419.3 g of 25% strength solution of styrene-acrylic acid copolymer, this initial charge then being heated to 85° C. with stirring under a nitrogen atmosphere.

Feed stream I:

-   384.8 g of styrene     Feed stream II: -   1.9 g of ammonium peroxodisulfate -   136.3 g of deionized water

When an internal temperature of 85° C. had been reached, feed stream I and feed stream II were metered continuously into the polymerization batch via two separate feeds, beginning simultaneously, over a period of 3 h 30, with stirring and retention of the reaction temperature. The pumps were flushed with 318.2 g of deionized water. After the end of both feed streams, the system was left to after react at the reaction temperature for a further 25 minutes. After that, the reaction mixture was cooled to room temperature and filtered on a filter having a mesh size of 160 μm.

The characterization of the copolymer obtained, in terms of solids content (SC) and average particle size (D), is given below:

-   SC=24.5% -   D=61 nm

Example 5 (Version with Colloid+Surfactant)

A 2 l reactor with stirrer and reflux condenser was charged with 1111 g of deionized water, 310 g of 25% strength solution of styrene-acrylic acid copolymer, and 3 grams of lauryl sulfate, this initial charge then being heated to 85° C. with stirring under a nitrogen atmosphere.

Feed stream I:

-   387 g of methyl methacrylate     Feed stream II: -   2 g of ammonium peroxodisulfate -   88 g of deionized water

When an internal temperature of 85° C. had been reached, feed stream I and feed stream II were metered continuously into the polymerization batch via two separate feeds, beginning simultaneously, over a period of 3 h 30, with stirring and retention of the reaction temperature. The pumps were flushed with 80 g of deionized water. After the end of both feed streams, the system was left to after react at the reaction temperature for a further 25 minutes. After that, the reaction mixture was cooled to room temperature and filtered on a filter having a mesh size of 160 μm.

The characterization of the copolymer obtained, in terms of solids content (SC) and average particle size (D), is given below:

-   SC=24% -   D=50 nm

Example 6 (Styrene-Methyl Acrylate Copolymer

A 2 l reactor with stirrer and reflux condenser was charged with 739.5 g of deionized water and 420 g of 25% strength solution of styrene-acrylic acid copolymer, this initial charge then being heated to 85° C. with stirring under a nitrogen atmosphere.

Feed stream I:

-   193 g of styrene -   193 g of methyl methacrylate     Feed stream II: -   2 g of ammonium peroxodisulfate -   136 g of deionized water

When an internal temperature of 85° C. had been reached, feed stream I and feed stream II were metered continuously into the polymerization batch via two separate feeds, beginning simultaneously, over a period of 3 h 30, with stirring and retention of the reaction temperature. The pumps were flushed with 318.2 g of deionized water. After the end of both feed streams, the system was left to after react at the reaction temperature for a further 25 minutes. After that, the reaction mixture was cooled to room temperature and filtered on a filter having a mesh size of 160 μm.

The characterization of the copolymer obtained, in terms of solids content (SC) and average particle size (D), is given below:

-   SC=30.0% -   D=70 nm

Example 7 (Styrene-Maleic Anhydride as Component B)

A 2 l reactor with stirrer and reflux condenser was charged with 400 g of deionized water and 750 g of 14% strength solution of styrene-maleic anhydride copolymer, this initial charge then being heated to 85° C. with stirring under a nitrogen atmosphere.

Feed stream I:

-   390 g of methyl methacrylate     Feed stream II: -   2 g of ammonium peroxodisulfate -   130 g of deionized water

When an internal temperature of 85° C. had been reached, feed stream I and feed stream II were metered continuously into the polymerization batch via two separate feeds, beginning simultaneously, over a period of 3 h 30, with stirring and retention of the reaction temperature. The pumps were flushed with 318.2 g of deionized water. After that, the reaction mixture was cooled to room temperature and filtered on a filter having a mesh size of 160 μm.

The characterization of the copolymer obtained, in terms of solids content (SC) and average particle size (D), is given below:

-   SC=29.6% -   D=70 nm

Example 8 (High Colloid Fraction)

A 2 l reactor with stirrer and reflux condenser was charged with 21.1 g of deionized water and 750 g of 25% strength solution of styrene-acrylic acid copolymer, this initial charge then being heated to 85° C. with stirring under a nitrogen atmosphere.

Feed stream I:

-   390 g of methyl methacrylate     Feed stream II: -   2 g of ammonium peroxodisulfate -   130 g of deionized water

When an internal temperature of 85° C. had been reached, feed stream I and feed stream II were metered continuously into the polymerization batch via two separate feeds, beginning simultaneously, over a period of 3 h 30, with stirring and retention of the reaction temperature. The pumps were flushed with 80 g of deionized water. After the end of both feed streams, the mixture was left to after react at the reaction temperature for a further 25 minutes. After that, the reaction mixture was cooled to room temperature and filtered on a filter having a mesh size of 160 μm.

The characterization of the copolymer obtained, in terms of solids content (SC) and average particle size (D), is given below:

-   SC=44% -   D=80 nm

Example 9 (Styrene-Acrylic Acid Copolymer with Tg of about 30° C.)

A 2 l reactor with stirrer and reflux condenser was charged with 433 g of deionized water, and 3 grams of lauryl sulfate (30% strength solution), this initial charge then being heated to 80° C. with stirring under a nitrogen atmosphere.

Feed stream I:

-   5 g of ammonium peroxodisulfate -   62 g of deionized water     Feed stream II: -   400 g of styrene, -   260 g of butyl acrylate, -   10 g of methacrylic acid, -   11 g of surfactant solution (lauryl sulfate, 30%), -   384 g of deionized water

When an internal temperature of 80° C. had been reached, feed stream I and feed stream II were metered continuously into the polymerization batch via two separate feeds, beginning simultaneously, over a period of 4 h, with stirring and retention of the reaction temperature. The pumps were flushed with 235 g of deionized water. After the end of both feed streams, the system was left to after react at the reaction temperature for a further 25 minutes. After that, the reaction mixture was cooled to room temperature and filtered on a filter having a mesh size of 160 μm.

The characterization of the copolymer obtained, in terms of solids content (SC) and average particle size (D), is given below:

-   SC=37% -   D=185 nm -   Tg=30° C. 

1. A method for inhibiting pitch deposition on pulping and/or papermaking equipment and/or machinery comprising adding to a pulp slurry containing pitch,. an effective amount of polymer dispersion comprising a component A and a component B, wherein component A being a homopolymer and/or copolymer of acrylic acid and/or one or more alkyl esters thereof, or methacrylic acid and/or one or more alkyl esters thereof, styrene and/or methylstyrene, vinyl acetate, itaconic acid, glycidyl methacrylate, 2-hydroxyalkyl (meth)acrylate, methacrylamide, N-hydroxyethyl(meth)acrylamide, dimethacrylate monomers, 1,3-butylene glycol dimethacrylate, ethylene glycol dimethacrylate, diethylene glycol dimethacrylate, propylene glycol dimethacrylate, dipropylene glycol dimethacrylate, 4-methyl-1,4-pentanediol dimethacrylate, divinylbenzene and/or trivinylbenzene, and component B being an aqueous solution of a styrene copolymer with acrylic acid, maleimide and/or maleic anhydride.
 2. The method as claimed in claim 1, wherein component A possesses a glass transition temperature >90° C.
 3. The method as claimed in claim 1, wherein component B possesses a molecular weight in the range from 3000 to 15 000 g/mol, optionally 3000 to 7000 g/mol.
 4. The method as claimed in claim 1, wherein the aqueous polymer dispersion is applied in combination with one or more calcium and/or magnesium salts, often naturally occurring in processing water, and/or component C, a cationic fixative, any of which promote coagulation of aqueous emulsion particles.
 5. The method as claimed in claim 4, wherein the aqueous polymer dispersion is pre-mixed with one or more calcium and/or magnesium salts or Component C, before adding the components to the fibrous slurry, during pulp and/or paper manufacture.
 6. The method as claimed in claim 4, wherein component C is selected from the group consisting of: polyethyleneimine (PEI), polydiallyldimethylammonium chloride (polyDADMAC), polyvinylamine (PVAm), polyaluminum chloride (PAC), zirconium salts, polyacrylamide (PAAM), polyamine and polyamideamine.
 7. The method as claimed in claim 1, wherein the aqueous polymer dispersion further comprises a component D in the form of a surfactant.
 8. The method as claimed in claim 1, wherein a water fraction of the aqueous polymer dispersion is 93% to 17.7%, preferably optionally 80% to 45% by weight.
 9. An aqueous polymer dispersion for coagulating and detackifying pitch in the processing of wood pulp and/or during the papermaking procedure, comprising a component A selected from a homopolymer and/or copolymer of the following group: acrylic acid and/or one or more alkyl esters thereof, or methacrylic acid and/or one or more alkyl esters thereof, styrene and/or methylstyrene, vinyl acetate, itaconic acid, glycidyl methacrylate, 2-hydroxyalkyl (meth)acrylate, methacrylamide, N-hydroxyethyl(meth)acrylamide, dimethacrylate monomers, 1,3-butylene glycol dimethacrylate, ethylene glycol dimethacrylate, diethylene glycol dimethacrylate, propylene glycol dimethacrylate, dipropylene glycol dimethacrylate, 4-methyl-1,4-pentanediol dimethacrylate, divinylbenzene and/or trivinylbenzene, and component B, which is an aqueous solution of a styrene copolymer with acrylic acid, maleimide and/or maleic anhydride.
 10. The aqueous polymer dispersion as claimed in claim 9, wherein the dispersion is present in combination with one or more calcium and/or magnesium salts, often naturally occurring in processing water, and/or component C, a cationic fixative, selected from the group consisting of: polyethyleneimine (PEI), polydiallyldimethylammonium chloride (polyDADMAC), polyvinylamine (PVAm), polyaluminum chloride (PAC), zirconium salts, polyacrylamide (PAAM), polyamine and polyamideamine.
 11. The aqueous polymer dispersion as claimed in claim 9, wherein the dispersion comprises a further component D in the form of a surfactant.
 12. The aqueous polymer dispersion as claimed in claim 9, which comprises 2% to 50%, optionally 5% to 30% of component A, 1% to 30%, optionally 3% to 10% of component B, 100 to 400 ppm calcium and/or magnesium carbonate (water hardness) 0% to 2% of component C, 0% to 0.3%, optionally 0% to 0.2% of component D, and 96% to 17.7%, optionally 90% to 45% of water (component E).
 13. An aqueous polymer dispersion comprising a component A and a component B, component A being a homopolymer and/or copolymer of methyl methacrylate, acrylate and/or styrene and component B being an aqueous solution of styrene copolymer with acrylic acid, maleimide and/or maleic anhydride, capable of being used for coagulating and detackifying pitch particles in processing of pulp and/or paper.
 14. A dispersion as claimed in claim 13, wherein the polymer dispersion is further combined with one or more calcium and/or magnesium salts and/or natural water hardness in pulp and/or papermaking process water and/or component C, a cationic fixative, optionally when waterhardness is below 15-20° dH.
 15. A disperions as claimed in claim 13, whereby the amount of aqueous dispersion has been applied to a cellulosic pulp slurry is in an amount of optionally 0.05 to 0.5%, or optionally 0.1 to 0.2%, based on dry weight of cellulose. 